Optical elements such as lenses, e.g. meniscus lenses, are normally circularly ground on the outside diameter. The lenses used in CO2 lasers are additionally usually decentered. The centering error for a 7.5″ lens is typically <0.1 mm after adjustment. If a lens is removed for cleaning, the position of the optical element relative to the holder is usually rotated after re-insertion due to the circular peripheral surface. The optical element must then be realigned (readjusted), since a centering from focus to nozzle better than 0.05 mm is required.
When workpieces are processed using a machine for thermal welding or cutting, in particular a laser processing machine, the cutting lenses can become contaminated. Impurities, deposits, or damage to the optics surface can result in increased absorption of the laser radiation. Consequently, the thermal loading of the optical element is increased. This ultimately results in a perceptible reduction in the laser power available in the processing region. In cases of severe contamination, in particular by spattering, the increased absorption of the laser radiation can lead to destruction of the optical element.
WO 99/59762 describes a device for checking the condition of a glass plate in a laser machining system with regard to contamination by dirt particles. The device comprises a holder for the glass plate. A temperature sensor in contact with the holder detects an increase in temperature of the holder. The increase in temperature is caused by increased absorption of radiation by the glass plate as a result of the dirt particles.